Cellulase producing bacteria were isolated from biogas slurry samples. Out of 24 isolates, JSR-001, identified as Stenotrophomonas maltophilia through 16S rRNA based molecular phylogenetic approach, showed maximum cellulase activity and it was selected for further enzyme production. Enzyme production was carried out using cheap and easily available agro-waste; AFEX-pretreated, rice straw as a growth supporting material. The cellulase enzyme was extracted and purified by Ammonium Sulfate precipitation (80%), dialysis (5kDa tubing cellulose membrane) followed by ion-exchange chromatography (Q-sepharose) and gel filtration chromatography using Sephadex G-200. The SDS-PAGE and zymogram analysis revealed the molecular weight of the produced cellulase enzyme to be ~38kDa. The optimum temperature and pH for enzyme activity were found to be 50^oC and 8.0 respectively. This purified cellulase enzyme having Km of 5.41mg/ml and a Vmax of 161.29 μmol/min/mg was found to be functionally stable even in the presence of 15% Sodium Chloride. It was also unaltered by ethanol, methanol, and isopropanol but partially inhibited by toluene and acetonitrile. The same was strongly activated by additives and detergents like Co²⁺, Na⁺, K⁺, Zn²⁺, SDS, Triton X-100 and Tween-20. Lignocellulosic substrates were also effectively degraded by this enzyme. Since the purified cellulase enzyme produced using our cheap substrate is halo and thermostable and also capable of degrading cellulosic substrates, it is apparent that it could be potentially considered in detergent, pulp and paper industries.